Field of the Invention
This invention relates to liquid metal cooled nuclear reactors, and more particularly to improved viewers using ultrasonic signals, which are submerged in the liquid metal, for detecting in a liquid metal cooled nuclear reactors the presence of a floating core component.
Background of the Invention
In one construction of a liquid metal cooled nuclear reactor, the reactor core, heat exchangers and coolant circulating pumps are submerged in a pool of liquid metal coolant. In operation of this nuclear reactor it is necessary to be able to locate and identify components submerged in the pool, and to do so before moving rotating shields in the roof of the pool in order to ensure that all the normally suspended absorber rods have been inserted in the core.
Television cameras are unsuitable for use in the opaque liquid metal to locate and identify components submerged therein. Therefore ultrasonic signals in the mega hertz frequency range have been used to produce a television-like visual display.
However, in the past difficulty has been experienced in the transmission of ultrasonic signals from a submerged transducer because the transducer must be protected from the high temperature environment of the reactor coolant and because core components do not always float up perpendicularly.
As diagrammatically shown in FIG. 1, a prior art submerged viewer utilizing ultrasonic signals is seen to consist of a transducer 10 which functions both as a transmitter and as a receiver. The transducer 10 is driven by a driving mechanism 11 installed in the center of a rotating shield 12. The submerged prior art viewer further includes a dip stick 13 by which the transducer 10 is suspended from the rotating shield 12. Transducer 10 can descend lower than a lower part 14 of upper core structure 15. Core components 16 which consist of core fuel assemblies, blanket fuel assemblies, removable shielding assemblies and the like are located under the upper core structure 15. The gap between the lower part 14 of the upper core structure 15 and the core components 16 is small, for example, about 50 mm.
Normally, none of the core components 16 float upwardly, but one of the core components 16 can be slightly floated upwardly by the pressure of liquid metal and the like. In order to prevent any floating core components from colliding with the lower part 14 of the upper core structure 15, the submerged viewer detects the floating core components. Namely, the ultrasonic signals which are transmitted from the transmitter of transducer 10 are reflected by the handling head 17 of the floating core components and the reflected ultrasonic signals from the handling head 17 are scanned by the receiver of transducer 10. Accordingly, when the viewer detects a floating core component, the action of the upper core structures 15, for example, control rod driving mechanism, are stopped to prevent the floating core components from colliding with the upper core structures 15.
Furthermore, as diagramatically shown FIG. 2, another prior art submerged viewer utilizing ultrasonic signals is seen to include a transducer 10 set aside from the center of rotating shield 12, and reflector plates 18 in the reactor vessel. The ultrasonic signals which are transmitted from the transmitter of transducer 10 are reflected by the reflecting plates 18 and the reflected ultrasonic signals from the reflector plates 18 are scanned by the receiver of the transducer 10.
Theoretically, prior art viewers submerged in liquid metal can detect floating core components effectively. However actually reflected ultrasonic sounds from the handling head 17 of the floating core components (in FIG. 1) or from the reflecting plates 18 (in FIG. 2) are not always received by the receiver of the transducer 10, for reasons hereinafter described. This is true because the handling head 17 of the core component 16 has, as shown in FIG. 3, a hexagonal cross-section. Therefore, the ultrasonic signals which are transmitted from the transmitter of the transducer 10 are not reflected to the receiver of the transducer 10 because of torsion or inclination of the handling head 17 owing to any thermal stress in the handling head 17 or to the pressure of liquid metal. Also, in the prior art embodiment of FIG. 2, the reflecting plates 18 are installed in the hot liquid metal and are so distorted or inclined owing to thermal stress therein so that the reflected ultrasonic signals from the reflecting plates 18 may not be scanned by the receiver of the transducer 10.